I'd say the check should still happen in the log function and a
better solution for unsupported browsers would be to use a separate
logging mechanism. Sure, you'll take a slight performance hit for
no good reason if you're using this in IE for now, but this is
strictly a development helper anyway.

I'd say the check should still happen in the log function and a
better solution for unsupported browsers would be to use a separate
logging mechanism.

That's exactly why this check should happen at runtime. I'm also
not fond at all of the idea of it silently failing. This is an
updating helper. If it fails silently. the developer might
understand it as meaning that his code is uptodate.